1. Field of the Invention
The present invention relates to a hair dryer, and more particularly, to a hand-held, ducted, axial-flow hair dryer.
2. Description of Related Art
There are myriad different approaches to providing hair dryers for consumer use. The primary consideration for such hair dryers is that they provide a flow of heated air in a sufficient quantity to evaporate water from the user's hair.
That goal is typically realized using a blower that directs air over a heating device, such as a resistance coil, and then to an outlet. Both axial-flow and centrifugal blowers have been used in known hair dryers. See, for example, U.S. Pat. No. 4,678,410 and German Patent DT 25 29 817, which disclose hair dryers using axial-flow impellers, and U.S. Pat. No. 3,943,329 and British Patent No. 1,519,652, which disclose hair dryers using centrifugal-flow impellers.
Hand-held hair dryers have been in general use for many years, and have found wide acceptance in the consumer market. As the market has matured, commercial success has demanded an increased ability to perform the hair dryer's main task, that is, drying hair, while providing a device that is quiet and safe to use.
To increase drying ability, one approach that will obviously work is simply to increase the heat of the air expelled from the unit. This approach has the drawback of increasing the possibility of burns to the user. There have been some attempts to ameliorate this shortcoming by providing ducting around the dryer outlet to inject ambient air into the exit air stream. See, for example, U.S. Pat. No. 3,284,611. U.S. Pat. No. 3,943,329 also discloses ducting provided around the hair dryer outlet for safety reasons. Hair dryers with this type of passive ducting do not have a significantly increased amount of fluid flow for drying a user's hair.
Therefore, to the extent that the use of such ducting reduces the risk of injury to the user, it also reduces the effectiveness of the exit air in drying the user's hair. That is, it reduces the temperature of the air directed against the user's hair without significantly increasing the amount of air available to perform drying.
A ducting arrangement is also shown in U.S. Pat. No. 5,317,815, in which a separate shell is attached to the outlet of a hair dryer. The shell contains an impeller vane that is rotated by the exit air from the hair dryer, and is said to induce ambient air into the flow through holes in the rear of the housing. Since the outlet of the shell is larger than the hair dryer outlet, the cross-sectional area of the air stream is increased. However, those familiar with the principles of fluid mechanics and the laws of physics will realize that driving the impeller vane with the exit air from the hair dryer imparts no additional energy to the air stream. Therefore, while it may marginally increase the amount of air flow, the increase is not significant of enough to offset the loss in drying effectiveness caused by reducing the air temperature through entraining ambient air in the flow.
Clearly, the amount of air flow can be increased simply by increasing the speed of the rotating blower. That, however, increases the amount of noise generated by the hair dryer. According to well known principles, so-called "dipole noise," N.sub.db, caused by rotating components satisfies the relationship: EQU N.sub.db .varies..omega..sup.6 (1)
From equation (1) it can seen that dipole noise is proportional to the sixth power of the rotational speed .omega. of the flow-generating components of a hair dryer. Therefore, very small increases or decreases in the rotational speed .omega. will have a great effect on the dipole noise generated by a hair dryer. Jet noise, generated by the air stream mixing with the ambient air at the dryer exit, also contributes to the noise perceived by the hair dryer user.
At the relatively low air flow velocities in a hair dryer, dipole noise is the predominant noise source. However, since jet noise scales with air flow velocity to the eighth power (that is, U.sup.8), jet noise can be reduced perceptibly by reducing the velocity of the air stream exiting the hair dryer. On the other hand, it is likewise important that the drying ability of the hair dryer not be compromised by reducing the air flow velocity.
It has been recognized that hair dryer dipole noise can be reduced by using an axial-flow impeller, with rotor and stator elements. See, for example, U.S. Pat. No. 4,678,410. And even a multi-stage axial-flow impeller, with successive rotor and stator stages, has been used. See, for example, German Patent No. DT 25 29 817.
However, those arrangements are used essentially to provide air flow like that provided by more widely used centrifugal blowers. They can produce the same air flow at a lower rotational speed of the blower, but they do not represent a different approach to solving the problems inherent with hair dryers using centrifugal blowers. That is, they can only produce significantly greater air mass flow by increasing rotational speed, and they can increase drying effectiveness only by increasing the heater (and therefore air) temperature.
What is required to move to the next generation hair dryer is a configuration that will provide optimum air mass flow and permit reduced air flow velocities, and also enable the efficient introduction of an appropriate amount of heat, while reducing noise levels to the barest minimum.